Most commercial fatty acids and glycerin are produced from natural fats, oils, and greases, including tall oil, vegetable oils and animal tallow. These fats, oils and greases jointly referred to as “natural oils”) comprise a mixture of mono-, di-, and tri-glyceride molecules, natural contaminants, and free fatty acids. While different natural oils include many chemically similar molecules, the natural oils vary in quality and each may be distinguished by its characteristic color, odor, chemical composition, and hydrocarbon chain length distribution, each of which is similar to that of the source of the oil. For example, sunflower oil is yellow; coconut oil has a sweet odor; and soybean oil is concentrated in long hydrocarbon chain lengths.
Some end-product applications require a quality in the natural oil that can only be achieved when processed from improved-quality feedstocks. A feedstock of “improved quality” will generally have reduced color and reduced odor compared with the raw feedstock; and some contaminants, including the naturally occurring free fatty acid, may have been reduced. The improved quality can be achieved by refining, bleaching and deodorizing; suitable processes include treating the oil with agents to coagulate gums in the oil and adsorption bleaching, wherein the oil interacts with a mixture of one or more adsorption agents, such as bleaching clay, activated carbon, and silica gel; and then the adsorbent with the captured odor, coloring agents and gums is filtered out. The presence of free fatty acid can be reduced by saponifying with metallic hydroxides and removal of soap stock by centrifuge or filter. Free fatty acids can also be reduced by vaporizing the free fatty acid off the non-volatile oil under vacuum. Odor is reduced by vaporizing the free fatty acid and other volatile odor bodies, which are then removed into the vacuum system. The resulting higher-quality, pretreated natural oils are commercially identified and specified under trading rules, such as those published by organizations such as the National Institute of Oilseed Products and the American Fats and Oils Association.
Conventional splitting processes are used to chemically transform the fats, oils and greases into crude fatty acid and crude glycerin. These splitting processes include the Twichell process, batch autoclave process, continuous hydrolysis and enzymatic process. The most prevalent of these technologies is continuous hydrolysis, which can be practiced using reaction-driving catalysts, though modern systems run non-catalytically. In the hydrolysis process, natural oils and water flow counter current through a high-pressure vertical column. High temperature and the removal of the glycerin product with excess water drives the reaction forward, splitting crude fatty acid from crude glycerin, which is removed with the water. The reaction is typically 95–99% complete with minor cross contamination coupled with some unsplit oil contaminant in the crude fatty acid and glycerin streams.
The “crude fatty acid” reflects the characteristic chain-length distribution, color, and odor of the feed oil. The crude fatty acid is flash or vacuum dried to be typically 97–99% fatty acid, the balance being 1–3% unsplit oil, water and other contaminants, including trace glycerin.
The glycerin stream is concentrated to remove excess water and settled to remove some unsplit oil and crude fatty acid. This concentration of the glycerin stream generates what is generally commercially described as “crude glycerin,” with 80–92% glycerin, 8–18% water and a balance of less than 2% unsplit oil and other contaminants including trace fatty acid. Crude glycerin is additionally produced during the direct saponification of natural oils into soap and glycerin, and by the trans-esterification of fats and oils into fatty esters and glycerin. With some pretreatment for salt and methanol or ethanol, the crude glycerin from the direct saponification process can be further refined with the same techniques as are used in the refining of glycerin that is split from natural oil by hydrolysis. For the purposes of this description, all sources of crude glycerin are processed according to the same methods.
The crude glycerin is further processed into commercial-grade refined glycerin. Refined glycerin is the commercial product for industrial, commercial or food-grade (United States Pharmacopoeia, USP) applications. The commercial applications for refined glycerin are wide, and include pharmaceuticals, cosmetics, explosives, personal care, tobacco and cleaning products. While there are some applications for refining glycerin that include ion-exchange technology, refined glycerin is generally processed from crude glycerin by distillation. Additional steps can include some pretreatment, such as water washing.
The basic principle of distillation is the separation of mixtures of volatile and non-volatile components. Separation occurs because the different components have different vapor pressures at constant temperature. The mixture is heated and then vaporized under vacuum, boiling off only the components that are volatile at that combination of temperature and vacuum. The non-volatile components, most contaminants, and some incompletely vaporized volatile components are extracted as “still bottoms.” Still bottoms are generally a low-grade byproduct, which is either recycled or disposed of into another application. The temperature and vacuum are selected to achieve a high yield, while limited to protect the quality of the distillate. Feedstock quality is a major factor in striking this balance. The distillate vapor is condensed as a clean product in cooling sections. Some extremely volatile components and non-condensable gases, such as air that was entrained in the feed, are passed on to the vacuum system.
In the case of glycerin distillation, the crude glycerin feedstock is heated to a temperature that vaporizes the glycerin and water, leaving the unsplit oil and contaminants, along with some unvaporized glycerin loss as still bottoms. The process is operated to improve the yield of the distillate while reducing the carry over of unwanted color and odor molecules. The degree of loss of some volatile glycerin is dependent on equipment design and rate of distillation; but, generally, the yield loss is 2–5% greater than the unsplit oil and contaminant level by mass. The distilled glycerin is collected in the condensing section, with the water passing to the vacuum system. The distilled-glycerin product is suitable for many industrial and commercial applications.
Certain applications for refined glycerin require color and odor quality that exceed what can be practically delivered by distillation alone. These refined glycerin products may require further color and odor treatment. Adsorption bleaching can be performed by either a batch or continuous slurry process, where powdered activated carbon is mixed with the distilled glycerin and then removed by filtration along with the captured color and odor components. The more common process is fixed-bed bleaching where the glycerin is flowed through a reactor vessel filled with an adsorbing material, such as granular activated carbon. The resulting product is suitable for higher-quality applications such as those calling for USP glycerin.
The crude fatty acid derived from the splitting process has a chain-length distribution, a color, and an odor that are characteristic of the feedstock oil. The crude fatty acid contains some unsplit oil from the incompleteness in the splitting process, contaminants from the process or feedstock, a small amount of glycerin, and a small amount of moisture. This is generally referred to as a “crude whole-cut fatty acid.” These whole-cut fatty acids will be differentiated both by the differences inherent in the feedstocks from which they were derived and/or by further processing. One way to group the main commercial fatty-acid product groups and their characteristics is into the following groups: whole-cut distilled, blended distilled, separated and fractionally distilled. Each of these groups is discussed, below.
Whole-Cut Distilled Fatty Acids:
Whole-cut distilled fatty acids are several fatty acids that are high-purity versions of their crude whole-cut intermediates, with their primary characteristics being derived from the feedstock. They can be further processed and differentiated by hydrogenation to change their degree of saturation.
A whole-cut distilled commercial product is produced by the distillation of selected crude whole-cut fatty acids. The characteristics of the crude whole-cut fatty-acid composition selected for distillation largely determines the characteristics of the finished product. The crude whole-cut fatty-acid composition includes mostly fatty acid with minor quantities of unsplit oil, water and contaminants.
Distillation is, as earlier described, the separation of ingredients based on differences in their vapor pressures at the same temperature under vacuum. The distillate that vaporizes is recovered in the condensing section as a clean refined product. Water and some highly volatile components are carried over to the vacuum system. The still-bottoms cut includes the unsplit oil, contaminants from the feed oil or picked up in the process, much of the color- and odor-containing molecules, and some volatile oil that was not vaporized.
The distillation process is operated to optimize yield of the distillate without carrying over unwanted color and odor molecules. The degree of loss of some volatile fatty acids is dependent on equipment design and rate of distillation; but, generally, the yield loss is 2–7% greater than the unsplit oil and contaminant level. Some distilled fatty acid may be additionally hydrogenated to reduce the number of unsaturated carbon atoms to both stabilize the fatty acid and to alter its characteristics, such as its melting temperature.
Distilled whole-cut fatty acids have a wide variety of applications in products such as soap, synthetic rubber, plastics, lubricants, cleaning products, recycled paper de-inking.
Blended Distilled Fatty Acids:
Combinations of whole-cut or component fatty acids can be blended to create characteristics of the mixture. This blending can be performed at various stages of the process.
Blended distilled fatty acids are merely the result of blending together whole cuts and/or fractions of whole cuts to create a product having the blended characteristics of the mixture. The reason for blending is to create characteristics unavailable from an individual natural oil. The blending can be carried out pre-process with the natural oils, with intermediates, or with finished fatty acids to achieve similar results. The process steps remain the same. The most notable application has been in the blending of long- and short-chain-length fatty acids to create desired lathering characteristics in bar soap.
Separated Fatty Acids:
Crude whole-cut fatty acids, particularly from tallow or palm oil, have mixtures of unsaturated (such as oleic, linoleic, and linolenic), and saturated (such as palmitic and stearic) chain lengths. Separated acids are those that take these crude fatty-acid feedstocks and separate the mixture into primarily unsaturated and saturated components by fractional crystallization. The separated acids are then further refined by distillation and, in the case of saturated acids, optionally by hydrogenation.
Separated fatty acids are mostly produced from crude whole-cut palm oil or tallow fatty acids because both feedstocks include significant quantities of both saturated and unsaturated chain lengths. Separation by differences in vapor pressures is impractical due to the fact that the vapor pressures of unsaturated and saturated versions of the same chain length are close. While separation can be accomplished by using means such as molecular sieves, separation is most-commonly performed by fractional crystallization. Separation is generally practiced with the aid of solvents, detergents and crystal modifiers.
The separation process is based on the wide difference between the temperatures at which saturated and unsaturated chain lengths crystallize. This temperature difference is exploited by carefully controlling the cooling of the mixture of saturated and unsaturated fatty acids. During the cooling process, the saturated fatty acid crystals form and grow while the unsaturated chains remain liquid. The crystallized saturates are then removed from the mixture by filtration. Some saturates pass through the filter, and some liquid gets trapped in the solid cake, creating some cross contamination. The saturate cake is remelted for further handling.
Both streams, although altered in their chain-length distribution, remain crude intermediates, with some evidence of the contaminants that were inherited from the crude whole-cut feed. The crude unsaturated fatty acid is further refined by distillation to make a commercial product suitable for applications in which the crude unsaturated fatty acid's low melting point and relatively high solubility are advantageous, such as for use as a liquid lubricant. The crude saturated fatty acid is further refined by distillation and usually hydrogenated to saturate the contaminant unsaturates from the separation process, converting the contaminant unsaturates into chemically equivalent saturated fatty acid. Commercial applications for the refined saturates include those applications where the high melting temperatures and the relative mildness of the saturates are advantageous, such as in solid lubricants and personal care products.
Fractionally Distilled Fatty Acids:
Crude whole-cut fatty acids, primarily from coconut and palm kernel oil, have broad chain-length distributions. Individual chain lengths have different vapor pressures at constant temperature. The relationship between the chain length and vapor pressure allows for the process of fractionation or fractional distillation to subdivide the chain lengths using targeted distillation and condensation temperatures. The bottoms cut from these operations are further refined by distillation and optionally by hydrogenation.
Fractional distillation, or fractionation, is a separation process based on the difference in vapor pressure of individual hydrocarbon chain lengths at the same temperature. Fractional distillation is widely practiced in the petroleum industry and elsewhere in the oleo chemical industry on fatty esters. Fatty acid fractionation, discussed here, is most commonly carried out on crude whole-cut coconut or palm kernel fatty acid, because of the wide distribution of carbon chain length in these fatty acids, primarily in the range of C-8 to C-18. The process concepts are also applicable to and practiced for use on separating, for example, C-20 and C-22 chain lengths. Most commonly, crude whole-cut coconut or crude palm kernel fatty acid is heated and sprayed into the lower part of a vertical vessel pulled under vacuum from its top section. The optimal temperature and vacuum condition are selected based on the targeted point of separation. The shorter chain fatty acids with higher vapor pressures boil and travel up the vessel.
The vessel design has one or more section(s) where vapors are condensed. The internal structure of the vessel creates a large contact area between single chain-length fatty acids in their liquid and vapor phases to facilitate purity of the separation, which is achievable at 99+%. The process creates a distillate top cut that vaporizes and condenses in the vessel, and an undistilled bottom cut that is drawn off. The distillate top cut may be of a single chain length, such as C-8 (with trace natural C-6), or of multiple chain lengths, such as C-8 and C-10 together; and a high purity in the top cut is achievable with trace presence of the higher-boiling-temperature C-12. In the case of a distillate of C-8 and C-10 together, the bottom cut would include the C-12 through C-18; and, noticeably, the bottom cut would also include all of the traditional still bottoms' unsplit oil and contaminants. The bottom cut is still considered a crude fatty acid because its contaminants are carried forward from the earlier splitting process. This bottom cut is henceforth referred to as “crude C-12/C-18.” Trace water and non-condensable gases pass to the vacuum system.
The crude C-12/C-18 is stored for future processing via this fractionator, or the crude C-12/C-18 is directly routed through a second fractionator. In either case, this second fractionation process is similar to the initial fractionation process, described above, except this second fractionation is carried out at a higher temperature of vaporization and at a higher condensing temperature profile to fractionate off the next top cut. For example, the top cut from this second fractionation can be either single-chain-length high-purity C-12 or a combination of C-12, C-14, etc., leaving a complementary crude C-14/C-18 or crude C-16/C-18 bottom cut. The process can be repeated, taking successive single or multiple cuts off the crude undistilled bottoms cut.
Any of the bottom cuts could be fully distilled at higher temperature profiles to turn a crude undistilled bottoms cut into a refined distilled product. That would leave the traditional still bottoms containing the unsplit oils, other contaminants and some vaporizable fatty acid. E.g., crude C-12/C-18 becomes C-12/C-18 distilled.
In each of the fractionation steps, the top-cut product is a clear commercial-grade product, and the bottoms cut is a crude intermediate material that is further processed for nearly all applications. Due to the unsaturation in some C-16 and C-18 chain lengths, sometimes distilled cuts that include those chain lengths are hydrogenated to reduce unsaturation. Fractionated products are used for applications that require certain specific chain lengths or that require the removal of chains that create unwanted characteristics in the derivative or its process.